Barcode scanner using an array of light emitting elements which are selectively activated

ABSTRACT

A bar code scanner employs an electronic means for causing the light beam to scan a bar code symbol, rather than using a mechanical device to generate the scan. A linear array of light sources, activated one at a time in a regular sequence, may be imaged upon the bar code symbol to simulate a scanned beam. Instead of a single linear array of light sources, a multiple-line array may be employed, producing multiple scan lines. The multiple scan lines may be activated in sequence, or activated simultaneously (time-division or frequency-division multiplexed. The multiple scan lines can provide signal enhancement, noise reduction or fault correction if directed to the same bar code pattern. Multiple scan lines may be generated using a single light source and a beam splitter, with mechanical scanning, as well as by the sequentially-activated light sources. Multiple simultaneous scan lines may be employed to generate a raster scan at lower mechanical scan frequency. In another embodiment, a tunable laser may be employed to provide a scan without moving parts; a laser beam from the tunable laser is reflected from a diffraction grating that produces an angular deviation dependent upon the wavelength of the laser output. As the frequency of the tunable laser is varied in some selected pattern, the laser beam will scan accordingly.

This application is a divisional of Ser. No. 864,367 filed Apr. 6, 1992,now U.S. Pat. No. 5,258,605, which was a continuation of Ser. No.493,134, filed Mar. 13, 1990 and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to bar code reader devices, and more particularlyto apparatus for generating a scanned light beam for use in reading barcode symbols.

Bar code readers are disclosed in U.S. Pat. Nos. 4,387,297, 4,409,470,4,251,798, and 4,760,248, all; assigned to Symbol Technologies, Inc. Thebar code readers described in these patents, as well as other devices ofthis type that are commercially available, usually employ oscillatingmirrors or similar mechanical means to generate a scanning pattern.While such devices are widely used in retail and other businessestablishments today and have been quite successful in accomplishingtheir objectives, there is nevertheless a continuing requirement toimprove reliability, reduce power consumption, reduce size and weight,lower the parts cost and manufacturing cost, and increase the speed andaccuracy of operation. One of the elements of the prior bar codescanners most susceptible to improvement along these lines is themechanical scanner device. The scanner devices may consist of a mirrormounted on a stepper motor; the mirror includes a flat portion to directthe outgoing laser beam and also a concave portion to collect reflectedlight and focus it upon a photodetector.

Bar code readers employ decoding circuitry to interpret the signalsproduced by a photodetector receiving the reflected light from the barcode symbol. Conventional decoding schemes rely upon data collected by asingle scanning spot moved linearly across the field where the bar codesymbol is located. The bar code data is embedded in background noise,and the decoding circuitry is more effective if the signal can beenhanced. To this end, faster scanning rates would permit theimplementation of multiple scans to increase reliability of the datacollected, but the mechanical scan generators previously used constrictthe speed and thus place limitations on the multiple scan approach.

It is the principal object of the invention to provide a bar code readeror the like that does not require mechanical devices such as oscillatingmirrors to cause a light beam to scan a symbol to be read. Anotherobject is to provide a bar code reader that is capable of faster scan,as by implementing the scan with no moving parts. A further object is totake advantage of fast scanning techniques to provide multiple scans tothereby increase the signal recovery ability, i.e, increase thelikelihood of recovering a valid decode of the bar code signal. Inaddition, the capability of providing multiple scans using a fast scanmethod permits improved facility for reading two dimensional bar codesymbols of the type having multiple rows of bar code patterns. Otherobjects include reducing the size, weight and power consumption of alaser scan type of bar code reader, as well as reducing themanufacturing cost and increasing the reliability and operating lifetimeof such devices.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a bar code scanneris provided that uses electronic means to cause a light beam to scan(move across) a bar code symbol to be read. The electronic means forscanning a bar code symbol is used in place of the conventionalmechanical device to generate the scan. In one embodiment, a lineararray of light sources, activated one at a time in a regular sequence,produces a rapid series of single-spot beams that are imaged upon thebar code symbol to simulate a scanned beam. Or, a tunable laser may beemployed, along with a diffraction grating that produces an angulardeviation dependent upon the wavelength of the laser output; as thefrequency of the laser is varied in some selected pattern, the laserbeam will scan accordingly. The speed of the scan can be much more rapidthan possible with mechanical scan devices, and the pattern of the scancan be adaptively adjusted to the particular symbol and position. Ascanner with no moving parts can provide an increase in reliability,although in some of the embodiments disclosed a scanning mirror isadvantageously employed. Improvements in power drain, size, shape andweight resulting from use of features of various embodiments of theinvention provide bar code reader devices of enhanced utility.

The faster scan allowed by employing no moving parts for generating thescan line permits multiple scans to be generated in the time of a singlescan in conventional equipment. This facility of providing multiplescans allows the signal to be decoded, i.e., the electrical input fromthe photodetector, to be enhanced. Multiple scans at vertically-spacedpositions on the bar code symbol, with the photodetector output being acomposite of the returns from all the scans, will be more likely toavoid erroneous readings due to defects in the symbol or backgroundnoise. Alternatively, the multiple scan lines may be used for readingtwo dimensional bar code symbols having multiple rows of bar-spacepatterns. Multiple scan lines may be implements with mechanical scangeneration such as an oscillating mirror, instead of with the scangeneration having no moving parts as discussed above, thus providing thesame advantages of signal enhancement and fault tolerance in signaldecoding. The multiple scan lines may be created by using multiple lightsources, or by employing a single light source and a beam splitter. Aliquid crystal device used as the beam splitter may dynamically changethe number of scan lines by varying the magnitude or frequency ofvoltage applied across the plates of the holding the liquid crystalmaterial.

According to another feature of the use of multiple scan lines in a barcode reader or the like, a raster type scan of a field may be generatedwith mechanical scanning elements, and, since multiple lines aregenerated for each scan, the frequency of the mechanical scanningelement may be reduced. The reflected light from the multiplesimultaneous scan lines is separated by first modulating each scan lineat a different frequency then using band-pass filters to recover thedifferent signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features characteristic of the invention are set forth in theappended claims. The invention itself, however, as well as otherfeatures and advantages thereof, will be best understood by reference toa detailed description of specific embodiments, when read in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a diagram in schematic form of a bar code scanner employing alinear array of light sources instead of a single light source andmechanical scanning, according to one embodiment of the invention;

FIG. 2 is an electrical diagram in schematic form of a part of thesystem of FIG. 1;

FIGS. 3a-3d are timing diagrams showing events vs. time for variousevents or voltages in the device of FIGS. 1 and 2;

FIG. 4 is an enlarged view of a part of the linear array of lightsources of FIG. 1, according to another embodiment;

FIG. 5 is a pictorial view, partly broken away, of the device of FIGS.1-4 mounted in a hand-held portable bar code reader unit;

FIG. 6 is a diagram in schematic form of a bar code scanner using nomoving parts according to another embodiment of the invention employinga diffraction grating;

FIG. 7 is a diagram in schematic form of a bar code scanner employingdual linear array of light sources instead of a single array as in FIG.1, according to another embodiment of the invention;

FIG. 8 is a timing diagram showing events or voltage vs. time forcertain occurrences in the system of FIG. 7 illustrating thecancellation of background noise;

FIGS. 9 and 10 are timing diagrams showing events or voltage vs. timefor certain occurrences in the system of FIG. 7 illustratingcompensation for faults in the bar code;

FIG. 10 is a timing diagram showing events or voltage vs. time forcertain occurrences in the system of FIG. 7;

FIG. 11 is an enlarged view of a bar code symbol being scanned by duallight beams, illustrating permissible tilt;

FIG. 12 is a plan view of a multiple-row bar code symbol being scannedby multiple scan lines according to another embodiment of the invention:

FIG. 13 is a pictorial view of a bar code scanner system correspondingto FIG. 7 but with two rows of light sources activated with signals ofdifferent frequency according to another embodiment of the invention;

FIG. 14 is a pictorial view of a bar code scanner system as in FIG. 7but employing a two-dimensional array of light sources according toanother embodiment of the invention;

FIG. 15 is a pictorial view of a bar code scanner system correspondingto FIG. 7, employing two scan lines, but using a single light sourcealong with a beam splitter;

FIG. 16 is a diagram of a bar code symbol scanned with three scan lines;

FIG. 17 is a pictorial view of a bar code scanner system as in FIG. 7for scanning a two-dimensional field, employing multiple simultaneousscan lines, according to another embodiment of the invention;

FIG. 18 is a diagram of a field including a bar code symbol, scanned bythe system of FIG. 17;

FIG. 19 is a diagram of a bit-mapped memory containing data recoveredfrom scanning a field such as that of FIG. 18 using the system of FIG.17;

FIG. 20 is a pictorial view of a bar code scanner system correspondingto FIGS. 7 or 15, employing multiple scan lines, using a single lightsource along with a liquid crystal device as beam splitter, according toanother embodiment of the invention; and

FIG. 21 is an elevation view in section of the liquid crystal device ofFIG. 20.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to FIG. 1, a bar code scanner according to one embodiment ofthe invention employs a linear array 10 of laser diodes or LEDs 11 thatare activated one at a time in sequence. U.S. Pat. No. 4,445,125 shows alinear array of laser diodes formed on a common substrate, as may beused as the array 10. The light output from the array 10 is focused by asuitable lens system 12 onto a line 13 at the focal plane of the lenssystem, i.e., the image of the array 10 will appear as a line 13scanning across the focal plane. A target such as a bar code symbol 14is scanned by the line 13, where the light beam scanning the line 13functions just as the laser scan used in more conventional bar codereaders. The advantage of the scanning arrangement of FIG. 1, however,is that there are no moving parts, and also the scan rate can be muchfaster than if limited by mechanical oscillating mirrors or the like.The spot size shown for the scan line 13 is merely illustrative; theactual spot size representing the images of the diodes 11 may be thesame size or larger than the minimum dimensions of the bars and spaces.

Light reflected from the bar code symbol 14 is focused upon aphotodetector diode 15 through a lens system 16, producing an analogelectrical output serially on a line 17, and this serial output isshaped via wave-shaping circuitry 18 to produce a square-wave type ofsignal on a line 19 to be then decoded in the usual manner to identifythe bar code symbol. A microprocessor device 20 is used to drive thearray 10 by way of a multiplexed drive circuit 21, and also used todecode the detected and shaped bar code signal on the line 19. The drivecircuit 21 may consist of a number of transistors 22 as seen in FIG. 2,each being connected in series with one of the diodes 11 of the array10; the base-emitter circuits of the transistors 22 are driven from adecoder 23 that receives a one-of-N address signal on lines 24 from themicroprocessor 20 to activate a particular one of the diodes 11; in thismanner, the diodes 11 can be turned on one at a time in a rapidsequence, starting at one end of the array and proceeding to the other.The pulse width used to drive the base of a transistor 22 to turn on adiode 11 may be variable by feedback based on reflected light, asdescribed below.

A monitoring photodiode 25 is positioned in the reader housing to detectthe overall light output from the light-emitting diodes 11, using asuitable optic arrangement, if necessary. The electrical output fromthis photodiode 25 is applied by a line 26 and a wave shaper ordigitizer 18a to an input of the microprocessor 20. This monitoringphotodiode 25 performs two tasks. First, the output power of the laserdiodes or LEDs 11 can be maintained in the proper range by adjustment ofthe pulse width of the driving current pulses, e.g., by an addressstrobe applied to the decoder 23 from the microprocessor 20 via a line27. Feedback to adjust the pulse width used to drive the transistors 22is thus provided. Or, the drive current for the diodes 11 could beadjusted by varying the level of the power supply to the diodes 11.Second, the monitoring photodiode 25 supplies an input to themicroprocessor 20 for use in fault detection and correction; if one ormore of the laser diodes 11 is faulty and does not produce any lightoutput, then the position or spot in the scan line 13 where this laserdiode was supposed to have illuminated is always dark and so isinterpreted as a black bar in the return signal even though the bar codesymbol 14 actually may have a white space in this position. To preventthis incorrect interpretation, the microprocessor 20 is programmed toignore or "blank" any signal on the input line 17 during the time slotof the faulty diode or diodes. Referring to FIG. 3a, the electricaloutput of the monitoring photodiode 25 on line 26 should be a continuousseries of overlapping or juxtaposed pulses 30, but where there is afaulty diode there will be a space 31, and so the microprocessorgenerates a blanking period 32 as seen in FIG. 3b. A bar code symbol 14as seen in FIG. 3c should return a signal 33 on line 17 as seen in FIG.3d, but the return during the blanking period 32 is false, so this inputis ignored or considered to be either black or white; if the code can bedecoded with this ambiguity, then a valid reading is possible, if not,then a false reading is at least avoided and another shot by the user,or an automatic rescan without user intervention, may produce a validreading. In any event, a fault can be signalled so the user can returnthe reader unit for repair.

As seen in FIG. 4, the optical system used to focus the array 10 uponthe focal plane of the bar code symbol 14 may employ a large number ofindividual lenslets 12a, 12b, etc., one for each laser diode 11, forcollimating the light from each diode. Each of the lenslets 12a, 12b,etc., is individually positioned to collimate the light for one diode,then a lens 12 is used to focus a single spot for each diode onto thefocal plane where the bar code symbol 14 is expected to be, to therebycreate the line 13 of spots as before. The array 10 may be formed of asemiconductor chip having a number of laser diodes or LEDs formedthereon, rather than by separate devices as illustrated; in such casethe physical size of the line of light sources 11 would be small and somagnification by the lens system 12 would be ample to create the desiredlength of the scan line 13. In order to create a smooth transitionbetween spots in the scan line 13 of FIG. 1, instead of the sequence ofdelineated dots illustrated in the enlarged view of scan line 13, theimage of the linear array 10 may be slightly defocused at the positionof the scan line 13.

Referring now to FIG. 5, the bar code scanner of FIG. 1-4 may be mountedin a portable hand-held unit 35 having a handle 36 gripped by the user.The laser scan beam 37 generated by the linear array 10 exits through awindow in the front of the unit, and the reflected light from the barcode symbol 14 also enters by this window to reach the photodetector 15.The microprocessor 20 and other circuitry of FIGS. 1 and 2 are mountedon a circuit board within the unit 35, as well as a battery if the unitis self-powered. The unit 35 is coupled to a central station by an RFlink or by a wire cable. A finger-activated trigger switch 39 isemployed to activate the scan, detect and decode functions when the userpoints the unit 35 toward a symbol 14.

The scanner of FIG. 1-5 can operate at very high rates. If there are Nlaser diodes 11 in the array 10, and T_(p), is the pulse width appliedto each laser diode via its transistor 22, the number of scans persecond is ##EQU1## and for N=100 and T_(p) =1μs the scan rate is 10⁴scans/sec. The value of T_(p) can be changed in real time to vary thescan rate.

Using the scanner device of FIGS. 1-5 with no moving parts, the scanpattern can be flexibly adapted; for example, if the bar code isrecognized to occupy only a fraction of the full scan line 13, power canbe conserved by applying drive current only to the transistors 22 forlaser diodes 11 that actually illuminate the bar code symbol 14 itself.Similarly, more scans can be applied to a small portion of the bar codesymbol by limiting the addressing applied to the multiplex drive circuit15 if decoding of the signal on line 17 indicates an ambiguity for thisportion.

Referring to FIG. 6, another embodiment of a bar code reader that has nomoving parts is illustrated. A tunable laser diode 40 or laser tubeproduces a laser beam 41 that passes through a suitable lens system 42to impinge upon a diffraction grating 43, from which the beam isreflected trough another lens system 44 to be focussed upon the plane ofa bar code symbol 45.

The diffraction grating 43 has the property of reflecting at an angledependent upon the wavelength of the light produced by the laser 40, soby varying the tuning of the laser 40 the beam 41 can be made to sweepacross the plane of the bar code symbol 45 along a line 46. Just as inthe embodiment of FIG. 1, light reflected from the bar code symbol 45 isdetected by a photodiode 47 that produces an analog electrical signal ona line 48 to pass through a wave shaper to a decoder. The assembly ofFIG. 6 can be mounted in a hand-held gun-shaped reader unit 35 as inFIG. 5, or in one of the reader housings of the type illustrated in theabove-mentioned patents, incorporated herein by reference. The readerunit usually has a trigger switch 39 operated by the user to activatethe laser source 40 and the photodetector 47 and its associatedmicroprocessor and data transmission circuitry.

Tuning the wavelength of the laser 40 by an amount Δλ causes a change inthe diffraction angle ΔΘ according to the relationship

    ΔΘ=(m/α cos Θ)Δλ

where m is the diffraction order, α is the line pitch (i.e., for agrating with 1200 lines/mm, αis 1/1200 mm or approximately 0.8 μm), andΘ is the angle between the incident beam 41 and a line normal to theplane of the grating 43.

The angular resolution as limited by diffraction is

    δΘ=(λ/N.α cos Θ)

where N is the total number of lines in the grating. The number ofresolvable spots is thus ##EQU2## For a 1-cm long diffraction grating 43with 1200 lines/mm, N=12,000. It is relatively straightforward toachieve Δλ˜50 Å for the tunable laser 40. Thus ##EQU3## For m=1, α cosΘ=0,5 μm, and Δλ=50 Å, the deflection resulting is

    ΔΘ=0.01 rad ≈0.6°

To get a scan angle of ±15°, the optical magnification needed via lenssystem 44 is approximately fifty.

The method of tuning the laser 40 to obtain a variable wavelength forthe output light beam 41 can be one several available. Semiconductorlasers can be wavelength tuned by various methods; some involvemechanical motion, and some do not require it. In any event, anelectrical input on line 50 from a sweep signal generator 51, as in theform of a sawtooth shaped waveform, causes the laser source 40 to varyin wavelength output, which results in generation of the scan line 46.

Referring to FIG. 7, another embodiment of the invention is illustratedwherein two of the linear arrays 10 and 10b are employed instead of onlyone as was the case in the embodiment of FIG. 1. The construction of theremaining parts of the system are the same as in FIG. 1. Use of twoarrays 10a and 10b provides two scan lines 13a and 13b, one above theother, separated from one another by a distance corresponding to thephysical separation of the arrays 10a and 10b and the magnification inthe optical system 12. This dual scan dine technique may beadvantageously employed in several ways. First, if the two rows of laserdiodes 11 in the two arrays 10a and 10b are activated in parallel, inthe same sequence, then the two scan lines 13a and 13b are likewise insync; in this case if the two scan lines traverse the same bar codesymbol 14, the reflected light received by the photodetector 15 is alsoin sync from the two scans 13a and 13b. The advantage of having two scanlines may be understood by reference to FIG. 8, where the backgroundareas 53 are seen to return uncorrelated signals 54, whereas the barcode symbol returns correlated waveforms from the two parts of thesymbol 14 being scanned by the two scan lines 13a and 13b. The singledetector 15 collects reflected light from the two scans at the same timeand sums the intensities of the reflections, so the contrast of theoverall signal 55 detected from the bar code 14 is enhanced. On theother hand, the areas 53 outside the bar code symbol 14 will result indifferent signals, and so the overall contrast from these areas isreduced. The digitizing circuitry used to shape the analog waveform onthe line 17 and recover the bar code information can more readilydistinguish the transitions in the bar code region of the signal fromthe uncorrelated returns from the areas 53. Referring to FIGS. 9 and 10,another advantage to the dual scans of FIG. 7 is that bar codeimperfections can be compensated for. If the bar code symbol 14 has adefect in the form of a gap 57 as seen in FIG. 9, then the signalreturned by the scan line 13b would have a corresponding false area 58whereas the return from the scan line 13a would be valid. The compositesignal 59 on the line 17 at the output of the photodiode 15 would stillbe able to be interpreted to recover valid data. Similarly, asillustrated in FIG. 10, if the defect is in the form of a black spot 60,the light return for one scan line will have a false area 61 appearingas if there was a very wide bar in the symbol, but the compositeelectrical signal 62 representing the sum of both scans 13a and 13b hasdistinct transitions and can be decoded to produce valid bar code data.

When two scan lines 13a and 13b are used as illustrated in FIG. 7, thescan lines should be perpendicular to the individual bars of the symbol14. The permissible misalignment depends upon the bar code density andthe amount of physical separation between the two scan lines 13a and13b. Referring to FIG. 11, assuming the diameter of the spot in the scanlines 13a or 13b to be larger than the minimum width D of a bar (orspace), the maximum permissible tilt angle α is given by

    tan α≈(0.5D)/L

where L is the separation between the two scan lines 13a and 13b.

In FIG. 7 an embodiment of the invention is shown having two arrays 10aand 10b, producing two scan lines 13a and 13b, but the number can belarger than two. Three or more scan lines provide the same types ofbenefits as just discussed, but to a greater degree. In addition,however, the capability of simultaneously scanning multiple bar codepatterns is available. With reference to FIG. 12, three scan lines 13a,13b and 13c are shown focused upon a bar code symbol 14a having threerows of patterns as set forth in U.S. Pat. No. 4,794,239. This is aCode-49 type of symbol that may have up to eight rows of patterns.Although only three scan lines 13a, 13b and 13c are shown in FIG. 12 forillustrative purposes, it is noted that many more scan lines may beadvantageously used in this embodiment of the multiple scan line featureof the invention. Previously, a single-scan reader was used torepeatedly scan the Code-49 type of symbol until an indicator light orbuzzer told the user that a complete and valid decode had been obtained.Or, as set forth in pending application Ser. No. 317,533, filed Mar. 1,1989, by Krichever & Metlitsky, assigned to Symbol Technologies, Inc.,incorporated herein by reference, the detector was a vidicon type ofimager instead of a single photodetector 15, and the symbol was notscanned but instead was illuminated in its entirety. The speed of thescan provided by this invention, however, allows the multiple scans ofmultiple rows to be distinguished merely by activating one row at atime, in a sequence of rows. That is, the laser diodes 11 for array 10afor scan line 13a are activated in sequence (while the diodes for allthe other arrays are deactivated), then the diodes for array 10b, thenfor array 10c, etc. Therefore, only one photodetector 15 can be usedsince at any given instant only one laser diode 11 is activated and thusonly one spot on the bar code symbol 14 is illuminated. Another way ofdistinguishing the reflected light so more than one scan line can besimultaneously activated is to generate light spots of differentwavelengths or modulated with different frequencies, then separate thereflected light by optical or electrical filters. As illustrated in FIG.13, for example, the individual laser diodes in one row may be activatedby a burst of a given frequency indicated by the waveform 64, instead ofby a rectangular pulse of constant voltage level, and diodes 11 inanother row activated by a similar burst but of a different frequency,then bandpass filters 65 responsive to these frequencies placed in thepath from the photodetector 15 to produce separate simultaneous inputsto the microprocessor 20 representing the returns from the bar codesymbol for the two different scan lines. Similarly, instead of frequencydivision multiplexing as illustrated in FIG. 13, time divisionmultiplexing may be employed wherein the light-emitting diodes 11 ofrows 10a and 10b are alternately activated, one at a time, then thereturn signal on line 17 is switched in synchronization with thealternate activation to produce two separated data streams; thisswitching can be done by the microprocessor 20 or by circuitry prior tothe microprocessor input.

In another embodiment of the concept of using scan generation with nomoving parts, and using multiple scan lines, the number of rows in thearray may be increased to create a complete raster-scan type of system.That is, as seen in FIG. 14, the array 10 may be a semiconductor chipcontaining a matrix of laser diodes or light-emitting diodes, perhaps(as an example) twenty-five rows by one hundred diodes per row (2500diodes in this example, although a larger number may be needed for ampleresolution), and the diodes activated one at a time; this arrangementcreates a two-dimensional field of view at the focal plane where the barcode symbol 14 is expected to be, similar to the two-dimensionalscanning features set forth in the above-mentioned copending applicationSer. No. 317,533. This arrangement of FIG. 14 is particularly adaptedfor use with multi-row bar code symbols such as employed in Code-49where rows of bar code characters are stacked one above the other.First, the position and orientation of the bar code may be found withinthe field 66 by a rapid but incomplete scan of the field (only perhapsone in five or one in ten of the diodes 11 activated) without attemptingto decode but merely looking for the general characteristics of bar codesymbols such as transitions per unit length. Then, only the localizedarea of the field 66 where the bar code symbol 14 is located is scannedwith activation of each diode in sequence, generating a set of scanlines positioned to intercept the rows of the bar code symbol 14. Notethat the angular orientation of the scan lines 13a, 13b, etc., need notbe parallel to the rows of diodes 11 but can be skewed to correspond tothe orientation of the symbol 14 within the field 66; a scan line wouldbe skewed by activating a sequence of diodes that are not necessarily inthe same row but instead define a straight line in the matrix of diodes.

Referring now to FIG. 15, a bar code scanner may produce dual scan lines13a and 13b as in FIG. 7, but, according to another embodiment of theinvention, the dual scan lines are produced by a single light source 68emitting a beam 69 that passes through a beam splitter 70 to generatetwo separate beams 71 and 72. The two beams are directed to a scanningmirror 73 driven by a motor 74, from which the beams are directed outthrough a suitable lens system 75 to impinge upon the bar code symbol 14as the two scan lines 13a and 13b. The assembly of FIG. 15 is mounted ina hand-held housing as in FIG. 5, or a stationary housing, as disclosedabove. The embodiment of FIG. 15 operates to provide improved resolutionand decoding as discussed above with reference to FIGS. 8, 9 and 10.

As disclosed above with reference to FIG. 7, the number of scan linesutilized in the embodiment of FIG. 15 is not limited to the two scanlines 13a and 13b illustrated, but instead may be more than two scanlines 13a, 13b and 13c such as illustrated in FIG. 12 for a twodimensional bar code symbol, or, as illustrated in FIG. 16, a number ofscan lines 13a, 13b and 13c may be advantageously used to scan aconventional bar code symbol 14 to obtain greater resolution, faultcorrection, etc. These three scan lines 13a, 13b and 13c are generatedby a single light source using a beam splitter and a mechanical scanmirror as in FIG. 15.

Referring now to FIG. 17, a bar code scanner system is illustratedemploying multiple simultaneous scan lines 13a, 13b and 13c repeated tocreate a raster type of scan. The scan is generated in this case usingmechanical means in the form of a mirror 76 rotated about a verticalaxis 77 to produce the horizontal traces and also rotated about ahorizontal axis 78 to produce the vertical movement of the three scanlines; there may be, for example, twenty horizontal traces (three scanlines per trace for a total of sixty) for each vertical frame, whereby afield 79 is scanned once each frame. Although three scan lines 13a, 13band 13c are shown in FIG. 17, it is understood that any number n of scanlines might be used, and the scanning frequency of the mechanicaldeflecting elements is reduced by a factor of n. The ability to generatemultiple scan lines during each cycle of oscillation or rotation of themirror 76 allows the requirements for construction of the mirror drivemechanism to be less stringent. That is, the drive mechanism can besmaller, lighter, less costly, and use less power, because it isoperating at one-third the speed. The dwell time of a light spot in ascan line 13a, 13b or 13c is also longer, allowing increased resolutionand/or lower power requirements for the light generators. Three separatelight emitting diodes or laser diodes 81, 82 and 83 are continuouslyactivated during a scan of the field 79 at three separate frequenciesf₁, f₂ and f₃ ; that is, the diodes are pulsed at three differentfrequencies in a manner similar to that illustrated in FIG. 13. As seenin FIG. 18, the three scan lines 13a, 13b and 13c make a trace from oneside of the field 79 to the other, then retrace to begin again at avertically-displaced position, as dictated by the indexing of the mirror76 about the axes 77 and 78. The three separate spots of light creatingthe scan lines 13a, etc., are modulated at the same frequencies f₁, f₂and f₃ as the light sources generating these spots, and so of course thereflected light from the spots is likewise modulated. A singlephotodetector 15 is responsive to light reflected from the field 79 forall three scan lines, and the analog electrical signal produced on theline 17 is applied to three band-pass filters 84, 85 and 86 tuned to f₁,f₂ and f₃, respectively, to recover the separate returns from the threescan lines 13a, 13b, and 13c. The outputs 87, 88 and 89 from the threeband-pass filters are demodulated to remove the modulating frequenciesand recover the envelope, and the demodulated signals applied to threeseparate wave-shaping or digitizing circuits 18 as before, then theshaped outputs 19 are applied as inputs to the microprocessor 20 fordecoding. As set forth in the above-mentioned copending application Ser.No. 317,533, the digital data at these three inputs 19 may be stored inrandom access memory and then accessed in patterns other than theoriginal raster scan pattern, so that if the bar code symbol 14 islocated in the field 79 at an angle then the effective scan lines forinterpreting the data in memory can be at the same angle. That is, thescan lines 13a, etc., need not be parallel to the pattern of the barcode symbol 14 to obtain a valid read and decode. Referring to FIG. 19,a bit-mapped image 79' of the field 79 stored in the memory 90 maycontain an image 14' of the bar code symbol 14 that is tilted andforeshortened due to the fact that the reader housing 35 of FIG. 5 inwhich the embodiment of FIG. 17 is mounted is not aligned normal to thesymbol. Thus, even though the data derived from line 17 is loaded intothis memory by the microprocessor 20 in a regular pattern of rows 13a',13b', etc., as the scan lines define the pattern, yet when accessed fordecoding it is necessary to create pseudo-scan lines 91, 92, etc., (bysuitable addressing of the memory) to generate signals for decode thatare aligned with the rows of the bar code pattern 14. It is noted thatthe scan lines 13a, 13b and 13c are shown to be adjacent one another inFIGS. 17 and 18, but instead an interlaced scan pattern could be used inthe raster scan of this embodiment.

The system of FIG. 17 employs three separate light sources 81, 82 and83, but a single light source could be used along with a beam splitter70 as in FIG. 15. In addition, an electro-optic light-modulating devicewould be placed in the path of each of the separate beams, downstream ofthe beam splitter. The light-modulating device would add a distinctivesignal to each beam so that it could be recovered by a band-pass filter84, 85 or 86 as before. The light-modulating devices may be, forexample, ferroelectric liquid crystal, electro-optic or acousto-opticcrystal gates.

Referring to FIG. 20, another embodiment of the invention is shownwherein multiple scan lines 13a, 13b, 13c and 13d are generated from asingle laser source 68 by a liquid crystal device 94. As in FIG. 15, thebeam 69 produced by the laser source 68 is split into multiple beams 95,and a mechanical device such as an oscillating mirror 73 moves thesebeams simultaneously across the area of the bar code symbol as themultiple scan lines 13a, etc. The liquid crystal device 94 has a voltageapplied to it by lines 96 and 97, and the magnitude and frequency of thevoltage determines the number of beams 95 produced and thus the numberof scan lines. In this manner, the beam 95 may be dynamically changedfrom a single beam to a split beam, by varying a voltage source 98. Forexample, the signal produced on line 17 from the photodetector 15 may beof poor quality using one scan line, but may be decodable using multiplescan lines as discussed above with reference to FIGS. 8, 9 and 10, sothe control program executed by the microprocessor 20 may cause thevoltage generator 98 to switch to a multiple scan line condition if avalid decode is not obtained with one scan line. Alternatively, thecontrol program may cause a switch from multiple scan to one scan lineif more intensity is needed, since the beam splitting would reduce thelevel of illumination with a constant output of source 68.

Referring to FIG. 21, the liquid crystal device 94 may comprise twoglass plates 99, each coated with a conductive film, and a liquidcrystal material 100 sandwiched between the conductive plates. Thematerial 100 may be, for example, a p-methoxy-benzylidene orp-n-butyl-aniline.

The liquid crystal device 94 of FIG. 20 is capable of spitting the beam69 into more than two beams, and so the voltage source 97 may applyvoltages of several levels to produce a selection of the number of scanlines needed, depending upon the conditions detected.

Although according to various features of the invention the scangeneration may use oscillating mirrors, the embodiments of bar codescanner devices as with no moving parts as described above have severaladvantages for some purposes, when compared to scanners that useelectromechanical components. First, the scan rate can be much faster,so the amount of time the laser is on can be reduced, lowering powerdrain. The faster speed will also allow a large number of scans to beaccomplished for one "read" operation, then the data signals correlatedwith one another using a suitable algorithm to increase the precision ofthe decode. Second, the scan pattern can be flexibly adapted, i.e., thescan can be tailored to fit the particular bar code symbol and position;e.g., after an initial scan it is determined that the location and widthof the bar code symbol in the field of view is at a specific place, andso the field is re-scanned at only this location, which will use lesspower. Third, after an initial scan there can be a re-scan of only asmall part that showed an ambiguous decode, attempting to get a validdecode of only the troublesome part. Fourth, improvements in reliabilitycan be provided in a device with no moving parts but instead implementedwith only electronic components and flexed optical devices.

In another aspect, the bar code scanner methods disclosed above whereinmultiple scan lines are employed provide other features of importance.The reliability of the decoding process can be enhanced by producing acomposite signal from multiple simultaneous scans, where the effects ofnoise or defects can be minimized. Or, the increased scanning speedpermitted by the use of no moving parts allows the multiple scans to besequential, one line at a time, which allows the reflected light fromthe multiple scan lines to be separated when using only onephotodetector; this arrangement permits scanning of multiple-row barcode symbols or the like.

While this invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asother embodiments of the invention and variations in the character ofthe disclosed or other embodiments, will be apparent to persons skilledin the art upon reference to this description. It is thereforecontemplated that the appended claims will cover any such modificationsor embodiments, or variations therein, as fall within the true scope ofthe invention.

What is claimed is:
 1. A bar code scanner, comprising:a) a scanning light beam generator for simultaneously producing a plurality of scan lines at different frequencies to impinge upon a bar code symbol or other symbol which encodes data using variations in light reflectivity, wherein said light beam generator includes an array of light emitting elements which are activated in a predetermined sequential order; b) a sensor, responsive to the scan lines reflected from the bar code symbol, for detecting light reflected from said bar code symbol and for producing an electrical signal representing data represented by said symbol; and c) a decoder which decodes the electrical signal produced by the sensor to generate a decoded signal indicative of the information contained in the symbol, wherein said decoder includes means for demultiplexing said electrical signal to recover separate signals for each scan line.
 2. A bar code scanner, comprising;a) a scanning light beam generator for simultaneously producing a plurality of scan lines to impinge upon a bar code symbol or other symbol which encodes data using variations in light reflectivity, wherein said light beam generator includes an array of light emitting elements, which are activated in a predetermined sequential order and wherein said scan lines are modulated at different frequencies; b) a sensor, responsive to the scan lines reflected from the bar code symbol, for detecting light reflected from said bar code symbol and for producing an electrical signal representing data represented by said symbol, wherein said electrical signal is filtered to separate responses to said plurality of scan lines; and c) a decoder which decodes the electrical signal produced by the sensor to generate a decoded signal indicative of the information contained in the symbol.
 3. A bar code scanner according to claim 2 wherein said elements are in at least one linear array and each linear array is activated in sequence beginning at one end and continuing in a regular pattern to the other end.
 4. A bar code scanner according to claim 2 wherein said multiple scan lines are repeated in a raster scan pattern.
 5. A bar code scanner according to claim 2 wherein said multiple scan lines are generated by different light sources.
 6. A method of reading a symbol comprising the steps of:(a) simultaneously directing a plurality of light beam scan lines modulated at different frequencies emitted from an array of light emitting elements to a target field where said symbol may be present; (b) detecting light reflected from said field from the plurality of scan lines; (c) producing an electrical signal corresponding to the reflected light from said plurality of scan lines; and (d) filtering the electrical signal to separate responses corresponding to respective ones of the plurality of scan lines.
 7. A method of reading a symbol according to claim 6 further including the step of decoding the filtered electrical signal to generate a decoded signal indicative of the information contained in the symbol.
 8. A scanner comprising:a) a scanning light beam generator for simultaneously producing a plurality of scan lines modulated at different frequencies to impinge upon a symbol which encodes data using variations in light reflectivity, wherein said light beam generator includes an array of laser diodes which are activated in a predetermined seguential order; (b) a sensor for receiving said light from said symbol and for producing an electrical signal representing data represented by said signal; and (c) a filter for filtering the electrical signal to separate responses to said plurality of scan lines.
 9. A bar code scanner according to claim 8 further comprising a decoder which decodes said electrical signal produced by said sensor to generate a decoded signal indicative of the information contained in said symbol.
 10. A bar code scanner according to claim 8 wherein said predetermined sequential order comprises turning on the first light emitting element in a row of said array, then turning off said first light emitting element and turning on the immediately adjacent light emitting element in the same row and continuing to turn off and turn on the adjacent light emitting element until the entire row has been traversed.
 11. A bar code scanner, comprising:a) a scanning light beam generator for simultaneously producing a plurality of scan lines at different frequencies to impinge upon a bar code symbol or other symbol which encodes data using variations in light reflectivity, wherein said light beam generator includes a plurality of light emitting elements; b) a mirror positioned between the scanning light beam generator and the bar code symbol for reflecting the scan lines toward the bar code symbol; and c) a sensor, responsive to the scan lines reflected from the bar code symbol, for detecting light reflected from said bar code symbol and for producing an electrical signal representing data represented by said symbol.
 12. The bar code scanner as claimed in claim 11 wherein the light emitting elements are light emitting diodes.
 13. The bar code scanner as claimed in claim 11 wherein the light emitting elements are laser diodes.
 14. The bar code scanner as claimed in claim 11 further including filter means for demodulating the electrical signal to recover separate signals for each scan line. 